Wednesday, December 23, 2015

Don't Panic!

Some of you may be wondering if your fruit trees are at risk
for breaking bud and growing with all this warm weather. Certainly, if it were
later in the winter, this might be the case. And, reports of (ornamental) cherry
bloom in the mid-Atlantic area (Washington D.C., and Newark, NJ), don't help. (Don't Panic!) We're not too worried about apples. Stone fruit chilling and grapes, however, are further along and with continued warm temperatures (with additional heat accumulation)
there is potential for injury, as winter is just beginning. A review of chilling, dormancy, and rest might help
explain what is going on.

First, the concept of dormancy and chilling hours/units. Deciduous trees, such
as apple and peach, gradually acquire cold hardiness and dormancy beginning in
the fall with cooler temperatures and decreased day length. They have adapted
such that they don't wake and start growing in early or mid-winter, because
obviously this would be a problem. If they broke (or swell) buds, started
growing, and then the temperature dropped below freezing, they would be toast.
In fact, this concept of endodormancy (aka “rest”) has been well studied and
quantified.

These chilling hours (or units as they are called when somewhat modified), in their most simple form,
accumulate when the temperature is between 32 and 45 degrees F (it is generally agreed). Some studies
have gone further and adjusted this accumulation based on other temperature
factors, such as when the temperature is above 45 but chilling units are still
accumulated (but at a slower or faster rate). For apples, it is generally accepted that
approximately 1,200 chilling hours between 32-45 are necessary before they
might start growing again. It is, however, somewhat variety dependent, with a
range of app. 800 (low chill varieties like ‘Anna’) to 1800 chilling
hours/units. Peach, however, has significantly lower chilling hours/unit requirements:
500-1,200, again highly dependent on variety. All this is important too in
warmer climates where sometimes not enough chilling hours/units are accumulated
to properly break bud and flower (and fruit).

Buds are most hardy (resistant to minimum winter cold
temperatures) when the chilling has not been close to met and temperatures have
gradually declined. This allows buds to increase in hardiness. Once the
chilling requirement has been met, it is the amount of heat units that accrue
that move buds forward to bloom. (But only after the chilling requirement has been met!) After chilling is met, temperatures above 32
degrees F. allow for internal development in the buds.

So, where do we stand in chilling hours/units right now? A pretty neat website, http://getchill.net, calculates them for you for
any location that has a Weather Underground weather station (http://www.wunderground.com/).
For the UMass Cold Spring Orchard (KMABELCH4) since November 1, 2015 (through
December 22), the calculation results:

Below 45 Model: 633 chill hours

Between 45 and 32 Model: 522 chill hours

Utah Model: 679 chill units

Zestar! flower bud, low risk for injury as chilling requirement has not been met (yet)

The important numbers are Between 45 and 32 Model (633 chill hours) and Utah
Model (679 chill units). Both well below 1,000. Apple buds aren't going
anywhere (yet). Peach buds, well, their chilling hour/unit requirements are
less (as low as 450). There could be an issue with stone fruit if the warm
weather persists (especially with some chilling still). Even though we might not have enough
warm temperatures to actually break bud, they will be more sensitive to
mid-winter cold once rest has been broken. I would not fret too much about it
yet, however, as most peach varieties we grow have fairly high chilling
hour/unit requirements.

Peach buds at higher risk because
chilling requirement is close to met;
in fact, these buds look swollen on 12/23/15

Second, and this is more problematic. Any time very cold temperatures --
particularly a rapid drop in temperatures -- following very warm temperatures during
the dormant season, can result in plant tissue damage. This is a cold hardiness
issue. Stone fruit are more sensitive to this because of their genetic
disposition to just stay “greener” all winter. Keep in mind that fruit tree
buds (and the cambium layer just under the bark) do not fully stop metabolizing
during the winter and are still quite “alive” despite their dormant appearance.
Very warm early or mid-winter temperatures followed by a rapid drop to extreme
cold is definitely not good news, although certainly it is not always a disaster either. (Just plain bad news.)

So that you might be able to sleep tonight, consider apple trees are
pretty well adapted to survive and grow despite the vagaries of weather.
Peaches (and other stone fruit) are less suited for surviving wild
temperature fluctuations, i.e., they are not as well adapted as apples to our wildly fluctuating weather here in central New England. Now, what the long-term effect of climate
change (warming) on our fruit trees is another question? (Weather is short-term,
climate is long-term. See Northeast and Northern Forests RegionalClimate Hub Assessment of ClimateChange Vulnerability and Adaptation andMitigation Strategies.) Ask me again in April how today’s “weather” will affect
our 2016 tree fruit crop. (Hint: we have a long way to go until apples and
peaches are harvested and in the cooler in 2016. And I might just tell you the answer is 42.)

Many thanks to my colleague Win Cowgill at Rutgers
University for helping me track down and understand the concept of chilling
hours/units and dormancy of deciduous fruit trees, as well as supplying some of
this text. You might also want to check out About Chilling Units and Hours from University of California Davis (although the emphasis is on low chill varieties suitable for normally warmer climates) and Cold Injury to Fruit Trees (a presentation I gave at the Ontario Fruit & Vegetable Growers's Conference in February, 2014.) And, if this is all too mind boggling, well there is always The Answer to the Ultimate Question of Life, the Universe, and Everything, which of course is...

1 comment:

I e-mailed the contact at getchill.net, asking him for references for the models if you ran the app on his site. His response below, good reading, particularly if you are not ready yet to settle for "42." :-)

*****Hi Jon,

Here are some of the references I used to compute the chill accumulation for each model. Hope this is what you are looking for.

I personally only use the below 45 model. There isn't a lot of information about ratings for fruit trees using the dynamic model. Utah models give wacky results in my warm winter climate (San Diego).

A reference for Utah, Positive Utah, and Dynamic models.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3077742/

A guide to calculate chill portions using the dynamic model.http://ucanr.edu/sites/fruittree/How-to_Guides/Dynamic_Model_-_Chill_Accumulation/

Some fruit trees rated in chill portions using the dynamic model.http://fruitsandnuts.ucdavis.edu/Weather_Services/chilling_accumulation_models/CropChillReq/

Here is the correct link for Utah, Positive Utah, and Dynamic models. I accidentally pasted the wrong one above.http://agis.ucdavis.edu/publications/2009/Sensitivity%20of%20winter%20chill%20models%20for%20fruit%20and%20nut%20trees%20to%20climatic%20changes%20expected%20in%20California's%20Central%20Valley.pdf